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best solid material that dissolves in water?

I need a solid material I can easy get, it should stay solid when exposed to air, for long periods, and when it comes into water, it should dissolve. What material would be the best choice?
edit:
It should stay firm for about 5 years. When in water, it needs to dissolve as quick as possible.

Hmm, this is a puzzler. On one hand, if solubility is more important (as you want the float to be released as quickly as possible) you could try something like either sugar or citric acid and baking soda (the active ingredients in Alka-Seltzer and bath bombs). However, you would sacrifice shelf life as I don't think it could be stored for 5 years (however, it's still cheap enough to manufacture repeatedly). On the other hand, if you're going for storage you could try the aforementioned salt, or perhaps gelatin, as they use in pills. It might take awhile to dissolve though. I'm still trying to figure out a happy medium (no pun intended). I'll check back soon if I think of anything.

That bath bombs idea is not bad, I'll think of it. thanks! I'm now testing with paper towel pills that are made from small wet paper towel pieces and dried. It doesn't dissolve in air because it doesn't dissolve in water, but in water it does tear apart(I think, I hope it will tear aopart enough, that's why I'm testing it)

Perhaps you could make pulp from wet toilet paper, then pack it around the float. Once it dries, it should stay hard, but in water I would imagine it would break apart really easily. Maybe mashing in some bath bomb would help expedite the process?

Great question, I've got a different use in mind but was researching the subject when I ran into this discussion. What I discovered was the pill gelatin requires the water to be a body temperature to work. In something like lake water it will just deform. Keep us posted on the paper test.

Thanks for your answer, I'm using it somewhat in the same way as the smugglers you described, I'm going to use it in a safety float for my camera. When it falls into the water I need to get it out quick enough to still be able to repair it by putting it in the oven for a while. That won't work in a lot of nature waters without a float, so I'm building that float. That's why the stuff needs to dissolve quick. About hwo quick would a half-centimeter layer of a 'salt lick' dissolve in water?

I'm not sure a salt lick is going to be the best source. They are very dense, unlike sugar cubes and there are no pores for liquid to get inside of it. They can survive being outside in the rain with only minimal melting. How about using paper as your fuse? I'm unclear about the actual deployment of your float, but there are a few ways you can use it to trigger something. One way would be to have a piece of metal held in place with paper. When the paper gets wet the metal will fall freely through the paper. Another way would be to tension a spring by hooking one end on a piece of paper. When it got wet the paper would tear and release the tension.

Paper! thanks for saying that because then I thought of toilet paper. You can make pills or balls from it if you make it wet and let it dry. That doesn't dissolve when it's exposed to air and it can handle some water drips, but whjen completely under water it breaks down. I'm testing it now with paper towel, I first cut it into small pieces and made a wet pill from it. Now it's drying. I think your idea with the spring would give too much stresson the paper, so after a long time also in the air it will tear. Thanks

Are you trying to build a float that stay stowed in a tiny package until it gets wet then deploys on contact with water. If it were me designing a system like that I would look toward using an electrical system with exposed contacts that would open a valve and allow a co2 ctg. to fill a bladder.